electrichild

amek m2500 eq Q pot...
« on: September 07, 2008, 02:41:02 PM »
Hello,

I'm realizing an EQ based on this schematic

http://www.gyraf.dk/schematics/amek_m2500_eq.gif

and just finished first HF section: frequencies' pot and gain pot work properly but Q pot (R7) must remain fixed otherwise moving it all goes into severe oscillation...any suggestion?

Thankz


12afael

amek m2500 eq Q pot...
« Reply #1 on: September 07, 2008, 03:44:56 PM »
check your wiring an values. it shouldn`t oscilate.

what op amp are you using?
heavy metal is the law!!!

electrichild

amek m2500 eq Q pot...
« Reply #2 on: September 07, 2008, 04:52:19 PM »
Quote from: "12afael"
check your wiring an values. it shouldn`t oscilate.

what op amp are you using?


Checkin the schems and your version (amek+harrisonFV2) in your old posts I found a little difference in the position of R11 1k (original schem) that is between pin2-C4 and R138 freq pot and your R6 1k that is between freq pot and pin7: can this cause the oscillation???
Other issue is extreme distorsion applying the maximum gain...
I'm using TL074CN opa
I retry check tomorrow

Thankz

12afael

amek m2500 eq Q pot...
« Reply #3 on: September 07, 2008, 11:25:33 PM »
I don`t think so, it is just a variable resistor.
check your resistors if you have extreme gain an error there could be the problem.
heavy metal is the law!!!

electrichild

amek m2500 eq Q pot...
« Reply #4 on: September 08, 2008, 03:38:02 AM »
Checking the circuit I found a bad solder point between the capacitor trough pin 1 and 2. All seems ok now but I notice something strange: after few minutes of properly working there is a drastically change in the level...too low. I retry powering up and all it's ok and again the level drops...
It's a long way...

gyraf

amek m2500 eq Q pot...
« Reply #5 on: September 08, 2008, 05:40:28 AM »
..check for HF oscillation..
..note to self: don't let Harman run your company..

electrichild

amek m2500 eq Q pot...
« Reply #6 on: September 08, 2008, 01:11:28 PM »
Quote from: "gyraf"
..check for HF oscillation..


How? :?

12afael

amek m2500 eq Q pot...
« Reply #7 on: September 08, 2008, 10:18:10 PM »
mmm is not so easy without a scope. The IC is unusually hot? maybe try a polyester cap between the ic supply pins and ground.

this is more a question than an answer, it`s possible to rectify the output and measuring DC to verify high-frequency oscillations?
heavy metal is the law!!!

electrichild

amek m2500 eq Q pot...
« Reply #8 on: September 09, 2008, 03:23:43 AM »
I placed 2 small caps (0.1uF) between + and - supply rails and ground...

 

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